The present invention relates to an apparatus and method for removing sulfur oxides from a flue gas produced during the combustion of a sulfur bearing fossil fuel. More particularly, it relates to an apparatus and method for controlling the temperature of the flue gas entering a spray dryer absorption chamber in order to permit greater amounts of sulfur oxide absorbent slurry to be sprayed into flue gas thereby allowing the treatment of flue gas containing high levels of sulfur oxide and insuring higher sulfur removal efficiencies.
Sulfur oxides are produced during the combustion of sulfur bearing fossil fuels, such as oil and coal. The adverse effect on the environment of discharging such sulfur oxide into the atmosphere is well established and has led to legislation greatly restricting the amount of sulfur oxide which may be emitted to the atmosphere from fossil fuel burning sources, particularly from coal fired power plants. As a result of such legislation, research and development efforts have centered on providing SO.sub.2 removal systems which can efficiently remove sulfur oxide form the flue gases of sources firing sulfur bearing fossil fuels, particularly high sulfur coals which are readily abundant in the U.S. One such promising system is commonly referred to as a dry SO.sub.2 scrubbing system.
A typical dry SO.sub.2 scrubbing system of the type designed for use on a fossil fuel fired power plant employs a spray dryer absorption chamber and a dry particulate collector disposed in series downstream of an air heater. In operation of such a system, the sulfur oxide bearing flue gases generated within the combustion chamber of the steam generator are first passed through an air heater in indirect heat exchange relationship with the incoming combustion air in order to extract heat from the flue gas to heat the incoming combustion air and thereby increase combustion efficiency. The flue gas leaving the air heater then passes through the spray dryer absorption chamber wherein a spray of sulfur oxide absorbent slurry contacts the flue gas and reacts with the sulfur oxides in the flue gas so that a major portion of the sulfur oxides are absorbed therefrom in the form of a dry particulate sulfur compound. The flue gas leaving the spray dryer absorption chamber is then passed through a dry particulate collector, such as an electrostatic precipitator, a cyclone separator or a bag filter, wherein the dry particulate sulfur compound and any other particulate matter, such as fly ash and unreacted sulfur oxide absorbent particles, are collected before the flue gas is released to the atmosphere.
In such a system, it is desirable that the temperature of a flue gas leaving the spray dryer absorption chamber be maintained above a preselected minimum temperature, specifically above the adiabatic saturation temperature of the flue gas, in order to insure that only dry particulate matter is entrained in the flue gas stream entering the downstream particulate collector. A known method of maintaining the flue gas temperature leaving the spray dryer absorption chamber above the adiabatic saturation temperature comprises adjusting the liquid feed rate to the sulfur oxide absorbent slurry sprayed into the spray dryer absorption chamber. The sensible heat in the flue gas entering the spray dryer absorption chamber is used to vaporize the liquid in the sulfur oxide absorbent slurry to produce a dry powder before the flue gas leaves the spray dryer absorption chamber. By varying the liquid feed rate, the amount of heat removed from the flue gas in vaporizing the liquid in the slurry as the flue gas traverses the spray dryer absorption chamber is also varied. Accordingly, for a given flue gas temperature entering the spray dryer absorption chamber, the temperature of the flue gas leaving the spray dryer absorption chamber can be adjusted by varying the liquid feed rate in order to prevent complete saturation of the flue gas thereby maintaining the temperature of the flue gas leaving the spray dryer absorption chamber above the adiabatic saturation temperature.
One problem associated with the above described method of controlling the temperature of the flue gas leaving the spray dryer absorption chamber arises when the flue gas to be cleaned has a high sulfur oxide content such as is typically the case when a high sulfur coal is combusted in a steam generating power plant. In such a case, the amount of sulfur oxide absorbent mixed with the liquid to form the sulfur oxide absorbent slurry must be increased in order to provide sufficient sulfur oxide absorbent surface to insure that efficient sulfur oxide removal is obtained as the flue gas traverses the spray dryer absorption chamber.
It is known that at high concentrations of sulfur oxide absorbent solids in the absorbent slurry, the slurry cannot be handled effectively from a fluid dynamic standpoint. That is, problems are encountered with the plugging of the spray nozzle, with slurry flowability, and with absorbent slaking. When high sulfur oxide content flue gas is to be cleaned in such a spray dryer absorption chamber, the liquid feed rate to the slurry must be increased in order to accommodate the higher level of sulfur oxide absorbent solids being added to the slurry. Consequently, a greater amount of liquid must be vaporized by the flue gas therefore the flue gas temperature leaving the spray dryer absorption chamber will drop. Thus control and optimization of the flue gas temperature leaving the spray dryer absorption chamber can no longer be obtained by varying the liquid feed rate.